[docs] added missing dependency

[DaVukovic]

Description

libunistring-devel is missing as a dependency for Fedora.

Motivation and Context

Can't build current Kodi master with only libunistring being installed.

How Has This Been Tested?

Tested on Fedora 30 and 31.

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[phunkyfish]

@lrusak you also built on fedora, maybe you could review?